(i) Field of the Invention
The present invention relates to a process for reducing the oxidation of food products.
(ii) Description of Related Art
When they are being manufactured, prepared or used, food products are liable to be exposed to ambient air, which may lead to oxidation of some of their components, degradation of their quality and shortening of their life. In particular, cooking oils which consist principally of fatty acid esters and glycerine, it being possible for the said esters to contain unsaturations on their hydrocarbon chain, in the course of time undergo reactions catalysed by light and heat, such as hydrolysis and oxidation, which lead to the breakdown of the said esters into fatty acids, into fatty acid peroxides and into other breakdown products; the effect which this has is to lower the smoke point of the oil baths, their coloration, and reduce their quality, and these themselves affect the taste qualities of the food cooked in them.
In order to prevent the degradation of food products or oils during their storage, transport or use, gas-based inerting of them using nitrogen is currently employed. The American patent published under number U.S. Pat. No. 5,298,271 describes a method of storing and transporting large quantities of palm oil consisting in permanently maintaining an oxygen-depleted atmosphere, that is to say an atmosphere enriched with nitrogen with a concentration in excess of 99.5%, above the oil baths, both after refining, during storage, during transfer to transport units and during transport. The formation of fatty acids, peroxides and other breakdown products is then greatly reduced. The authors have shown that the method is effective only if the maximum oxygen level in the atmosphere above the baths is less than or equal to 1% by volume; otherwise, there is a strong increase in the proportion of fatty acids and the proportion of peroxides. However, this method, as well as the one described in the European patent application published under number EP 0580896, are not satisfactory when small-scale applications are involved, such as the purification of a small volume of gas above a food bath of a few litres, principally because of the constraints connected with the scale of these methods.
Another solution consists in inerting not only the atmosphere above the food baths, but also in injecting nitrogen into the bath, irrespective of whether the latter is in operation or off. The process described in the international patent application published under number WO 95/27401 consists in injecting, using small injection nozzles with a diameter of the order of 5 .mu.m, nitrogen in divided form into the industrial frying bath containing about 1.5 to 5 m.sup.3 of oil. Analyses of these baths show that with inerting under nitrogen, whether in operation at 180.degree. C., at a nitrogen flow rate of 5 to 10 m.sup.3 /hour or when off at room temperature, at a flow rate of 2 m.sup.3 /hour, the proportions of free fatty acids and of peroxides are much lower. Currently, the system most frequently employed is partial inerting of the vats by flushing.
Other methods described in the literature employ the absorption technique by circulating the liquid to be preserved through a filtration module containing various kinds of absorbents. These absorbents may contain antioxidants impregnated on various supports; the use of active carbons covered with antioxidants is described in the American patent published under number U.S. Pat. No. 5,348,755; the use of magnesium oxide impregnated on active carbon is described in the American patent published under number U.S. Pat. No. 4,125,482; the use of aqueous solutions containing EDTA, ethylenediaminetetraacetic acid, is described in the American patent published under number U.S. Pat. No. 4,968,518; the use of solutions of citric and ascorbic acids is described in the American patent published under number U.S. Pat. No. 5,068,115. A process involving settling may also be cited, which is described in the American patent published under number U.S. Pat. No. 5,008,122 and consists in mixing the frying oils and solvents having different specific gravities. The mixture formed is then separated by gravity, one of the flows, containing the solvent and the impurities, is discarded while the other, consisting of clean oil, is returned to the frying vat.
In general, all the processes described above are appropriate only for large volumes of food products, for example more than a cubic metre, and are difficult to adapt to small installations such as oil baths in fast-food establishments. This is because they require, in particular, the installation of gas injectors and hoods, the creation of a perfect seal of the vat/hood system, the fitting of gas lines, and systems for controlling the gas flow rates, and generate problems involving storage of the cylinders.
The Applicant Company has therefore tried to develop a process and a device which make it possible to minimize the oxidation of food products while being simple to employ and suitable for small installations, such as those in restaurants.